In the fields of PET and SPECT imaging, a radiotracer is administered to a patient which is preferentially uptaken by particular regions of the body. The radiotracer causes the emission of gamma photons which are detected by the medical imaging system and used to generate images of the radiotracer's spatial distribution. Such images may subsequently be interpreted by a physician in order to investigate the functioning of biological processes. The quality of these images, particularly their signal to noise ratio, is desirably improved in order to assist in clinical diagnosis and is in part dependent upon the sensitivity with which gamma photons are detected.
The detection of gamma photons is carried out by a gamma camera in a SPECT imaging system. A gamma camera comprises one or more detector heads that are positioned to receive gamma photons from an imaging region. Each head comprises one or more gamma photon detectors. In contrast to SPECT, in a PET imaging system gamma photons are detected in pairs by modules of gamma photon detectors disposed radially about an imaging region. A gamma photon detector is therefore a common feature in both SPECT and PET imaging systems and is defined herein to comprise a scintillator element in optical communication with an optical detector. An optical detector is defined herein to comprise an optical sensor that receives optical radiation and generates an electrical signal in response to said optical radiation.
In a gamma photon detector a scintillator element creates a pulse of scintillation light when struck by a gamma photon. The associated optical detector subsequently converts the scintillation light into an electrical signal. In seeking to maximize their image quality, imaging systems desirably use sensitive gamma photon detectors which efficiently convert a received gamma photon's energy into an electrical pulse. Maximizing this efficiency therefore demands that the optical detector captures as much of the original scintillation light produced by the scintillator element as possible.
A further improvement is achieved in PET and SPECT imaging systems by improving the optical isolation between neighboring gamma photon detectors. Such imaging systems typically have a densely packed arrangement of gamma photon detectors in which light leakage between scintillator elements risks the misinterpretation of its source, thereby degrading their spatial resolution.
Known methods for improving the capture of scintillation light by the optical detector in a radiation detector include the wrapping of the scintillator element in for example PTFE tape. A small air gap between the PTFE tape and the surface of the high refractive index scintillator element acts to retain scintillation light within the scintillator element that is incident to its surfaces at oblique incidence angles using total internal reflection. The PTFE tape operates to return some of the scintillation light to the scintillator element whose incidence at near-normal incidence angles means that it is not otherwise retained by total internal reflection.
Another method disclosed in U.S. Pat. No. 5,091,650A involves the application of inwardly-reflecting layers to the surfaces of the scintillator element other than those in optical communication with the optical detector. These improve both the capture efficiency of scintillation light by the optical detector, and also the optical isolation between neighboring scintillator elements.
In document Simulating Scintillator Light Collection using Measured Optical Reflectance SCH-TNS-00249-2009.R1, Janecek et al discuss the need to accurately model the optical properties of reflecting layers applied to scintillator elements in predicting the light collection from a scintillating crystal and furthermore disclose models for reflecting layers such as Lumirror®, ESR film, Tyvek®, and TiO paint.
Patent application WO2012/153223 discloses to mitigate light trapping in a scintillator crystal by roughening at least one side of a plurality of pre-formed polished scintillator crystals, and further to apply a specular reflector material to the roughened crystals which are arranged in an array.
US patent U.S. Pat. No. 6,369,390B1 discloses a scintillation camera crystal having a plurality of light scattering holes in the crystal extending toward the photo-sensor and communicating with at least one surface of the crystal. The crystal is formed from a first material and the holes include a second material differing from the first material for deflecting the light generated by the scintillation crystal in response to incident gamma rays and reducing the spread of the generated light.
In the gamma photon detectors described above, whilst the use of inwardly-reflecting layers improves the capture efficiency of scintillation light by the optical detector, much of the scintillation light is still not captured by the associated optical detector. This degrades the gamma photon detector's signal to noise ratio and furthermore degrades the image quality of SPECT and PET imaging systems employing such detectors. Consequently a need exists to improve a gamma photon detector's sensitivity.